Peel and roughly chop potatoes. Place in a saucepan, cover with cold salted water and bring to the boil. Simmer until just past tender. Drain and pass through a potato ricer, or mash. Add butter to the hot milk and cream. Return mashed potato to the heat and add milk, cream and butter, stirring until the mixture is smooth and creamy – not all the liquid may be needed, depending on how starchy the potatoes are. Add nutmeg and chopped herbs and season to taste with salt and pepper. If you decide on the luxury of truffle oil, add it just before serving, with a final whisk.